In the prior art image separation techniques, areas near edges of characters are mistakenly recognized as white areas when the characters are printed on non-white background of a certain shade of color. This because the character image data is processed by an edge-enhancement filter to restore clarity of the characters. However, when the background of the character image is non-white, due to the edge-enhancement filter, the areas near the character edges are affected to have an intensity level that is lower than the one before the edge-enhancement, and the lowered intensity level reaches a level of white color. Prior art image separation techniques for determining white regions often mistakenly consider these lowered intensity areas near character edges as white regions. Thus, the non-white background of a certain color is mistaken as white. Although the above described mistaken white regions are not an obvious eye sore if they are continuous, when characters have discontinuous areas, the above described mistaken white regions become eye sore as they appear randomly and hapharzardly around characters on a non-white color background.
To avoid the mistaken white region extraction, the relevant prior art techniques have used predetermined matching pixel patterns. On the other hand, these predetermined matching pixel patterns are insensitive to real white areas where there is a high-density of lines within a character. The predetermined matching pixel patterns are not able to distinguish line-dense portions in characters and also avoid the selection of these areas as white areas.
Relevant prior art techniques for edge detection used data that is highly sensitive to dark color or high intensity such as an intensity level and the green color component (G). Since the above data is not highly correlated to color, light colors especially yellow (Y) is not easily detected. As opposed to the above use of a single color component, other prior art techniques utilize a minimal value of each of the color component signals such as red (R), green (G) and blue (B) outputted by a scanner to extract image data characteristics. The use of the three color components such as R, G and B is susceptible to various factors such as the measurement location and ambient light. When a measurement location is not identical, edges tend to be emphasized. Especially due to a dot pattern screen angle of printed matter, the above described influence is significant. Although an edge-enhancement filter is applied to restore the blurred edges, the edge-enhancement filter should be color-component specific.
Japanese Patent Office Publication or JPOP02-294884 discloses an image separation technique to determine character regions based upon the lowest level color component image data. JPOP02-295357 discloses an image separation technique to determine a dot pattern based upon the lowest level color component image data.
JPOP 10-23251 discloses an image separation device and technique to tertiorize image data to extract white regions and black regions and the extracted regions are further compared to a set of predetermined matching patterns to select line image edges.
JPOP10-108012 discloses an image separation device and technique to detect edge areas, dot pattern regions, white background regions, and based upon the above detection, the regions are determined as a part of a character or a picture.
JPOP03-64251 discloses a control technique in which an operator modifies a SEG signal to determine a scanning direction intensity change detection level T1, a sub-scanning direction intensity change detection level T2, a diagonal scanning direction intensity change detection level T3 and a dot pattern determination level T4 for determining a character/picture determination so as to adjust priority for the character processing or the photograph processing based upon the character/picture determination.
JPOP05-292312 discloses a prior art technique to detect a dot pattern peak pixel and to determine a calculated value for distribution of the dot pattern peak pixel in a predetermined size area. The calculated value is compared to a threshold value to determine whether or not the predetermined size area is a dot pattern region. JPOP09-247481 discloses a prior art technique for dot pattern determination for each of C, M and Y image data.
Japanese Patent No. 2856867 discloses a prior art color processing device for applying single black color in a black character region that is a picture area which is surrounded by black character edge regions.
JPOP07-95397 discloses a prior art device to detect edges and high intensity regions in order to prevent the blurring of small characters and white regions inside thick lines of high-intensity thick characters. The prior art device recognizes that high-intensity regions surrounded by edges within a predetermined distance is a character region. The prior art device provides an appropriate process for each of the recognized regions.
It is desired that an image data processing technique should improve the quality of the reproduced image in separating the character portions and the areas near or surrounding the character portions. In particular, as the result of the character edge enhancement, it is desired to substantially reduce the possibility for mistakenly processing white areas around the character edges as well as to improve the correct detection rate for character edges in high line density areas so as to increase reliability in the reproduced image data. One of the factors in obtaining the above improvements is to select an appropriate mode for character data and graphics data. For example, the graphics mode is used for character or dot pattern screen image data that is on a light color background such as yellow.